The following relates generally to wireless communication and more specifically to adaptive modulation orders for multi-user superposition transmissions (MUSTs) with non-aligned resources.
Wireless communications systems are widely deployed to provide various types of communication content such as voice, video, packet data, messaging, broadcast, and so on. These systems may be capable of supporting communication with multiple users by sharing the available system resources (e.g., time, frequency, and power). Examples of such multiple-access systems include code division multiple access (CDMA) systems, time division multiple access (TDMA) systems, frequency division multiple access (FDMA) systems, and orthogonal frequency division multiple access (OFDMA) systems (e.g., a Long Term Evolution (LTE) system). A wireless multiple-access communications system may include a number of base stations, each simultaneously supporting communication for multiple communication devices, which may be otherwise known as user equipment (UE).
CDMA, TDMA, FDMA, and OFDMA systems may communicate with multiple UEs through the use of resource sharing and/or orthogonal transmissions. In some cases, separate communications to multiple UEs may be accomplished by strategically sharing resources or by orthogonally transmitting to the UEs over simultaneously-shared (“common”) resources. For instance, a TDMA system may designate time intervals for transmissions during which a UE is scheduled to receive a transmission over a common frequency channel—e.g., the base station may transmit to a first UE in a first time interval, a second UE in a second time interval, etc. An FDMA system may simultaneously communicate with multiple UEs by sending UE-specific transmissions over separate frequency resources allocated to each of the UEs.
CDMA systems may simultaneously transmit to each of the UEs using the same time and frequency resources, but may uniquely modulate transmissions to different UEs with an orthogonal code. The UEs may be assigned unique orthogonal codes, and may apply the orthogonal codes to received signals to identify the transmission intended for that UE. OFDMA utilizes a combination of TDMA and FDMA techniques applied over orthogonal subcarriers. In some cases, multiple-input multiple-output (MIMO) techniques may be employed, which take advantage of spatial properties of channels to the UEs to separate data streams sent over different spatial resources. For example, MIMO techniques include modulating transmission streams with space-time orthogonal codes, such as spatial frequency block codes (SFBC). These spatial resources may be called spatial layers, and the same or different streams of data may be transmitted over different spatial layers. For single-user MIMO (SU-MIMO), multiple spatial layers are transmitted to the same UE, while in multiple user MIMO (MU-MIMO), multiple spatial layers are transmitted to different UEs.
A wireless communications system may utilize MUST techniques to support communications with multiple UEs that share time and frequency resources without using orthogonal transmissions. For example, a MUST transmission may include multiple streams of data intended for multiple UEs using common resources—e.g., at least partially overlapping time, frequency, and/or spatial resources—but may transmit the streams of data without uniquely orthogonalizing the transmissions to the different UEs. MUST transmissions may take advantage of the physical locations of the UEs in the wireless communications system to transmit multiple streams of data intended for multiple UEs. The different streams of data may be transmitted over different non-orthogonal transmission layers. In some cases, the base station may transmit an enhancement layer to a first UE that has relatively higher geometry (e.g., higher signal-to-noise ratio (SNR), closer to the base station) and a base layer to a second UE that has a relatively weaker geometry (e.g., lower SNR, farther from the base station) using overlapping resources. MUST may also be referred to as non-orthogonal multiple access (NOMA).
The MUST transmission layers may be multiplexed in various ways including by using different transmit power levels, hierarchical modulation, or other multiplexing techniques. Hierarchical modulation may describe a scenario in which a base modulation order of a base layer and an enhancement modulation order of an enhancement layer are combined into a joint symbol constellation. In some cases, the resources occupied by a base layer in a transmission may not align with the resources occupied by an enhancement layer in the same transmission. Efficiently using resources for a transmission with unaligned transmission layers presents challenges.